Synovial Sarcoma

Synovial sarcoma is a rare cancer of soft tissue that can occasionally arise in the pleura or chest wall. Under the microscope it can look very much like sarcomatoid or biphasic mesothelioma, so it is one of the main conditions pathologists must rule out. It is not caused by asbestos, and a specific gene change confirms it.

Origin

The tumor was named in the early twentieth century because it was thought to come from synovium, the lining of joints. That turned out to be wrong; it can arise almost anywhere. In the 1990s researchers found that nearly all cases carry a characteristic swap between the X chromosome and chromosome 18, joining the SS18 and SSX genes.

Significance in Asbestos Context

Synovial sarcoma is most common in adolescents and young adults, though it can occur at any age. When it arises in the chest, it can form spindle-cell or two-pattern (biphasic) tumors that resemble mesothelioma, and both can stain for some keratins. Pathologists separate them with molecular tests, such as FISH or sequencing for the SS18-SSX fusion, and with stains like TLE1 or newer fusion-specific antibodies. The distinction matters to patients because treatment differs, and it matters to claims because synovial sarcoma is not an asbestos disease. In litigation, defense pathologists sometimes suggest a sarcoma as an alternative diagnosis to sarcomatoid mesothelioma, so a confirmed molecular result, in either direction, is valuable evidence. If the fusion test is negative and the tumor spreads along the pleura in the pattern of mesothelioma, that result helps support the mesothelioma diagnosis.

Example Sentence

Because the patient was 32 and the tumor was a spindle-cell mass in the chest wall, the pathologist ordered SS18 fusion testing, which confirmed synovial sarcoma rather than mesothelioma.

Related Terms

Sarcoma, Sarcomatoid, Biphasic Mesothelioma, Spindle Cell, Fluorescence In Situ Hybridization (FISH), Translocation, Differential Diagnosis, Mesothelioma Misclassification

Notes

• If a spindle-cell chest tumor is diagnosed, ask whether molecular testing was done to rule out a sarcoma.
• Ask for the full pathology report, including any consult reports from outside experts.
• A second opinion at a center that sees many sarcomas and mesotheliomas is reasonable for unusual tumors.